The invention relates to a novel manganese dioxide catalyst which can be used for the hydrolysis of organic nitrites to the corresponding carboxamides, and to a process for preparing the catalyst. The invention also relates to a catalytic process for hydrolysing organic nitrites to the corresponding carboxamides with the aid of the catalyst.
The invention relates in particular to a catalytic process for hydrolysing 2-hydroxy-4-methylthiobutyronitrile to 2-hydroxy-4-methylthiobutyramide, a valuable intermediate in the preparation of 2-hydroxy-4-methylthiobutyric acid, the hydroxy analogue of methionine (MHA), and salts thereof. These substances find use as an animal feed additive, in particular in poultry breeding. These methionine-like compounds can replace methionine and significantly improve the utilization of proteins in the feed.
The hydrolysis of 2-hydroxycarbonitriles (cyano-hydrins) is a special case of nitrile hydrolysis. It is not possible to use any of the known processes for hydrolysing nitrites, in which strong bases may be used, because a back reaction of the cyanohydrin to aldehyde and hydrogen cyanide proceeds under these reaction conditions.
The 2-hydroxy-4-methylthiobutyronitrile can also be hydrolysed with highly concentrated mineral acids, preferably with sulphuric acid, in a virtually equimolar amount. In the first reaction step, the amide of the substituted butyric acid forms. An industrially readily realizable separation of 2-hydroxy-4-methylthiobutyramide and sulphuric acid, with the aim of being able to reuse the sulphuric acid, is, however, not known. Only after the hydrolysis of the amide to the hydroxycarboxylic acid is the mineral acid removed as ammonium hydrogensulphate, and worked up back to sulphuric acid in an additional, expensive process step.
It is also known that manganese dioxide catalyses the hydrolysis reaction of carbonitriles to amides, as described, for example, in DE 1593320.
As a result of the incorporation of manganese of other valence states into the crystal lattice, the stoichiometric composition of natural and synthetic manganese dioxide is in the range between MnO1.7 and MnO2.0. Extraneous ions such as sodium, potassium may be present in the crystals. Manganese dioxide exists in several allotropic modifications. They differ greatly in their behaviour as a catalyst. The crystallinity is at its most marked in pyrolysite (beta-manganese dioxide), the most stable modification. This form is catalytically inactive. The crystallinity is less marked in the further modifications and extends up to an amorphous product, ramsdellite. It is possible to assign the modifications by x-ray diffraction. Some of the chemically and catalytically active forms of manganese dioxide are hydrated and additionally contain hydroxyl groups.
Numerous patents describe catalytic processes for hydrolysing carbonitriles, especially 2-hydroxynitriles (cyanohydrins), with manganese dioxide. These processes are highly suitable, for example, for hydrolysing acetone cyanohydrin, as shown in U.S. Pat. No. 4,018,829, with which yields of over 90% are achieved in the hydrolysis of acetone cyanohydrin to 2-hydroxybutyramide with the aid of manganese dioxide.
The catalytically active modifications of manganese dioxide are, however, also active as oxidizing agents, which significantly restricts their use in the hydrolysis of thioether- or thiol-substituted nitrites, caused by their easy oxidizability. This reduces tetravalent manganese partly to trivalent manganese, and correspondingly oxidizes the sulphur.
DE 1593320 describes a process for hydrolysing nitrites to amides with the aid of manganese dioxide, in which yields up to over 90% were achieved with aliphatic nitrites. Only 8% yield of amide was achieved with thiodipropionitrile, which makes it clear that conventional manganese dioxide is hardly suitable as a catalyst in the presence of easily oxidizable thioether groups.
EP 0 597 298 describes a partial reduction of manganese dioxide by a pretreatment with reducing agents such as alcohol in order to improve the catalyst properties, in particular to suppress oxamide formation there. With increasing proportion of trivalent manganese oxide, however, the activity of the catalyst declines.
The oxidizing action of manganese dioxide is generally undesired in the hydrolysis of nitrites which bear easily oxidizable groups such as thiol or thioether groups. In particular, the S-oxidization is undesired in the hydrolysis of 2-hydroxy-4-methylthiobutyronitrile to 2-hydroxy-4-methylthiobutyramide, an important intermediate in the preparation of the animal feed additive 2-hydroxy-4-methylthiobutyric acid. Oxidation of the sulphur forms the sulphoxide and this ultimately reduces the yield of 2-hydroxy-4-methylthiobutyramide. This by-product formed by oxidation cannot be removed without a considerable level of cost and inconvenience and then leads to a contaminated end product which can no longer be used directly as an animal feed additive.
The reduction of the catalyst which is associated with the oxidization of the sulphur also shortens its lifetime, which, in an industrial process, leads to economic disadvantages, such as increased catalyst consumption and regeneration complexity with corresponding costs.
The patent JP 09104665 describes the preparation of active δ-manganese dioxide and defines its activity via the parameter of surface area. The hydrolysis of 2-hydroxy-4-methylthiobutyronitrile with full conversion is also described with this catalyst. There is no discussion whatsoever of the formation of sulphoxide in this publication on the part of the applicant. By virtue of application of the conditions specified there, it was, however, found that, during the reaction, the sulphoxide of 2-hydroxy-4-methylthiobutyronitrile is formed up to more than 4% in a selectivity of at least 1.6% (comparative example, Example 11), which is highly disadvantageous. Moreover, the nitrile conversion was only 96.1%, the amide yield 79.8% in continuous mode.
The same applicant describes, in the patent EP 0 731 079, a process for preparing carboxylic acids by hydrolysing cyanohydrin with the aid of manganese dioxide and subsequent hydrolysis of the amide formed with alkali to give the salt of the carboxylic acid. The electrodialysis then separates the carboxylic acid and sodium hydroxide solution from one another. In Example 3, in a column reactor with δ-manganese dioxide (amorphous) not described in more detail, the formation of MHA amide at 50° C. by hydrolysis of 2-hydroxy-4-methylthio-butyronitrile with a nitrile conversion of 100% is reported at an MHA amide yield of even 100%. It was likewise impossible to confirm this result. Instead, it was found that, especially in a column reactor with high catalyst concentration, the oxidizing action of active manganese dioxide comes to bear. With simultaneous reduction of manganese4+, this leads to catalytically inactive Mn3+ and to increased formation of the sulphoxide. The reworking of this example which is quite similar to JP 09104665 also shows that the sulphoxide of 2-hydroxy-4-methylthiobutyramide is formed with a selectivity of approx. 2% to more than 4%.
The patent FR 2 750 987 solves the problem of the oxidation of sulphur by coating silicon dioxide with manganese dioxide. However, the catalyst contains only 5 to 10% of active catalyst constituents. This must be balanced out by the use of a large amount of catalyst or by reaction times of 17 to 45 hours. For an industrial process, this procedure is disadvantageous.